Tromaville High School, tucked beside a nuclear power plant in New Jersey, sits under the shadow of a facility that keeps a dangerous accident under wraps. The plant’s owner, Mr. Paley, fights to keep the facility open rather than risk a shutdown by the safety commission. The incident triggers a radioactive water leak that seeps into the school’s drinking fountain and gruesomely kills a student named Dewey, Arthur Lorenz. The school’s infamous troublemakers, a gang known as The Cretins, were once part of the honor society, and the tale suggests the plant’s runoff has helped turn them into violent psychopaths that terrorize the campus. They harvest leaves from a radioactive marijuana plant growing near the plant and sell them to Eddie Harold, James Nugent Vernon.
That night, at Eddie’s indoor bikini beach party, Eddie Harold, James Nugent Vernon pressures his friend Warren Gil Brenton and Warren’s girlfriend Chrissy Murphy Janelle Brady into trying the radioactive joint. The joint is dropped and trampled before anyone else can sample it, but the mutated drug proves to be a potent aphrodisiac. Warren and Chrissy end up having sex in Eddie’s loft, only to be haunted by disturbing nightmares of grotesque mutations, though those effects fade by morning.
Some time later, Chrissy discovers she is pregnant and spits out a small creature into a toilet. The creature travels through the building’s water pipes and lands in a barrel of radioactive waste, where it mutates into a larger, more menacing form. The nuclear plant orders a lockdown of the school and launches an investigation into Dewey’s death. One of the nuclear workers ventures into the basement to find signs of a spill; his equipment detects nothing obvious beyond a foul odor, but a second sound he had dismissed lures him closer. As he peers into a barrel, the monster’s arm reaches out and claws his face before killing him.
Meanwhile, Warren, exhausted by The Cretins’ harassment, experiences a radiation-fueled rampage that leaves two of the gang dead, with no memory of the event afterward. The Cretins, expelled from the school and cut off from their customer base, attack the principal and force him to trigger the school’s Radiation Alarm to evacuate, then take control of the building. They capture Chrissy as bait for Warren and hold her in the basement, planning to kill her in front of him—only to be interrupted by the arrival of the adult monster.
Warren enters the school to rescue Chrissy and confronts the adult monster, which proceeds to kill the rest of The Cretins. He fires a laser from the physics laboratory to knock the creature down as he and Chrissy flee the building. Mr. Paley enters the room moments before the school explodes, killing himself and the creature in a final, devastating act. The students celebrate as the loudspeakers announce that the school will be shut down for remodeling. In the wreckage of the destroyed building, another small creature wriggles into view, signaling that the nightmare may not be over.
